1. Field of the Invention
The present invention relates to a vibration damping device. More particularly, it relates to an improvement structure of a vibration damping device which is suitably used as an engine mount, in which an inner fitting and an outer fitting disposed separately on the outside of the inner fitting are connected to each other by a rubber elastic body substantially having a T shape as a whole.
2. Description of the Related Art
As a vibration damping connector or a vibration damping support interposed between members constituting a vibration transmission system, a vibration damping device having a construction such that an inner fitting attached to one member and an outer fitting attached to the other member, which is arranged separately on the outside of the inner fitting, are connected to each other by a rubber elastic body has been used to shut off or reduce vibrations transmitted between both of the members. To such a vibration damping device, not only a vibration load from one direction but also vibration loads from other directions are applied. Therefore, various constructions capable of responding to a plurality of vibration applications have been studied.
As one of vibration damping devices having such constructions responding to the plurality of vibration applications, US 2005/0217918 A1 proposes an engine mount having a construction such that in a vibration damping device arranged so as to match the axial direction of the vibration damping device to the right and left direction of a vehicle because of a vehicle space etc., an inside element, which is an inner fitting, and a cylindrical outside element, which is an outer fitting, arranged separately on the outside of the inside element are connected to each other by a rubber elastic body; the rubber elastic body is forked into two branches on the outside element inner peripheral surface side by a void extending in the axial direction to take an inverse V shape; and further the void is extended to a side rubber elastic body connecting a flange portion provided on one end side in the axial direction of the inside element to a flange portion of the outside element provided so as to face the flange portion of the inside element so that a spring constant in the right and left direction (axial direction) of the vehicle can be set independently of the vertical and longitudinal spring constants of the vehicle, by which the ratio (spring ratio) of three-directional spring constants (Ks) in the up and down direction, front and rear direction, and right and left direction of the vehicle can easily be designed so as to have a desired value. Even in such a mount construction, a load in the vertical direction, which is the up and down direction of the vehicle, can be supported mainly due to compressive deformation action by the inverse V shape of the rubber elastic body, so that a relatively large spring constant can be realized. However, in the horizontal direction, which is the front and rear direction of the vehicle, the inverse V-shaped rubber elastic body supports an applied load by means of shear deformation, so that it is difficult to increase the spring constant. Therefore, it is remarkably difficult to regulate great oscillatory motion in the front and rear direction of the vehicle in a large-size power plant (engine). In order to regulate such oscillatory motion in the front and rear direction of the vehicle, it is necessary to further increase the spring constant (Ks) in the horizontal direction of the mount corresponding to the front and rear direction of the vehicle. Moreover, it is necessary to realize the increase of the horizontal spring constant while the spring constant in the up and down direction of the vehicle is kept stable.
On the other hand, JP-U-B-6-20679 and JP-A-2001-191800 propose an engine mount having a construction such that a rubber elastic body connecting an inner fitting to an outer fitting consists of a pair of rubber arm portions, which are positioned so as to extend in the horizontal direction in almost equal thickness on both right and left sides with the inner fitting disposed therebetween to connect the inner fitting to the outer fitting in the horizontal direction, and a rubber leg portion, which is separated via a currant portion (cavity portion) provided between the paired rubber arm portions and the rubber leg portion, which supports the inner fitting and the outer fitting in a connecting or contacting manner in the up and down direction (vertical direction) perpendicular to the horizontal direction, so that the rubber elastic body substantially has a T shape. In this construction, the rubber elastic body is only intended to regulate and absorb a relative vertical vibration (load) between the inner fitting and the outer fitting. The paired rubber arm portions provided on both right and left sides of the inner fitting merely take shearing stresses, and are not considered as a construction for supporting a vibration (load) applied in the horizontal direction. Therefore, this engine mount has a problem that the usage durability thereof is poor.
In the above-described mount having the construction such that the inner fitting and the outer fitting are connected to each other by the rubber elastic body having a T shape as a whole, when a vibration load is applied in the horizontal direction and acts on the paired rubber arm portions, which are the head portion of the T-shaped rubber elastic body, stresses concentrate in the vicinity of a connecting portion of the rubber arm portion to the inner fitting. Therefore, in this construction, a crack and a fracture etc. are liable to be generated, which causes a problem of deteriorating the durability of rubber elastic body. Also, in this construction, in the rubber leg portion, which is the leg portion of the T shape of the rubber elastic body, as well, in the case where the inner fitting and the outer fitting are connected in a substantially equal thickness or merely connected so that the side surface assumes a linear trapezoidal shape, the rubber leg portion may be buckled if a high load is applied in the up and down direction. In addition, it is difficult to sufficiently absorb the deformation of the rubber leg portion. Therefore, this construction has a problem of generating a crack and a fracture etc., and thereby the durability of the rubber leg portion is deteriorated.